Capabilities of terahertz super-radiance from electron bunches moving in micro-undulators

An available frequency range of powerful coherent radiation from sub-picosecond and picosecond bunches with high charge and comparatively moderate particle energy of 3 - 6 MeV that are formed in laser-driven linacs significantly extends if one uses a micro-undulator. Such an undulator with a helical symmetry and a high transverse field can be implemented by redistributing a strong uniform magnetic field by a helical ferromagnetic or copper insertion. According to simulations and experiments with prototypes, a steel helix with a period of (8-10) mm and an inner diameter of (2-2.5) mm inserted in the 3T-field of a solenoid can provide a helical undulator field with the same periodicity and an amplitude of about of 0.6 T. Using a more complex hybrid system with a permanently magnetized helical structure can increase this value up to 1.1 T. The necessary helices can be manufactured on the machine, assembled from steel wires, formed from powder placed into a hollow helical shell or 3D - printed. Simulations based on the WB3D code demonstrate that using such undulators with the length of (30-40) cm enable single-mode super-radiance from bunches with charge of 1 nC and duration of 2 ps moving in an over-sized waveguide in frequency range of 3-5 THz. The calculated efficiency of such process many times exceeds efficiency that can be obtained with short bunches of the same initial density.